<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>border-radius</title>
<style type="text/css">
div.circle{
    height:100px;/*与width设置一致*/
    width:100px;
    background:#9da;
    border-radius:50px;/*四个圆角值都设置为宽度或高度值的一半*/
    }
/*任务部分*/   
div.semi-circle{ 
    height:100px;
    width:50px;
    background:#9da;
    border-radius:50px 0 0 50px;
    }
   
</style>
</head>
<body>
<div class="circle">
</div>
<br/>
<!--任务部分-->
<div class="semi-circle">
</div>

</body>
</html>